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This network will bridge two very active disciplines in physics, namely the quantum electrodynamics of atoms or ions strongly interacting with light in resonators, and the emerging field of solid-state superconducting circuit quantum electrodynamics. Advanced techniques will be developed jointly with industry partners for the manipulation of a deterministic number of particles - atoms, ions or artificial atoms - with electromagnetic fields covering the microwave and the optical frequency spectrum. The interdisciplinary training of a new generation of young researchers will strengthen the European expertise in those fields, and will allow for a new discipline to emerge that combines single-atom control methods with superconductor micro-chip fabrication. The use of high-quality resonators, whether superconducting transmission lines or highly-reflecting mirrors, coupled to a controlled number of particles will open novel avenues to explore quantum dynamics via hitherto inaccessible physical mechanisms. These new control scenarii will be strengthened by the development of potentially marketable technologies of great multidisciplinary interest.

The network groups 10 research centres and 3 companies representing the cutting edge of research in the quantum electrodynamics of fundamental systems in Europe. The network will train 12 ESRs and 2 ERs, with focus on (i) establishing bonds between solid-state and quantum optics physics, (ii) strengthening the communication between theory and experiment, and, (iii) concretizing links between fundamental and applied research. Prominent scientists and industry leaders will contribute to the schools and workshops. Special attention will be given on developing complementary skills, such as communication, presentation, project planning and management.

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Quantum electrodynamics receives a boost in Europe

An EU initiative brought together researchers from the solid-state electronics and atomic physics fields to investigate the fundamental interaction between light and matter. Researchers developed common theoretical understanding to support experiments and address new phenomena.

The term cavity quantum electrodynamics (QED) was initially coined to describe the coupling of real atoms to microwave or optical photons stored in a resonator. Circuit QED also includes the investigation of such phenomena in the solid state with artificial atoms coupled to on-chip superconducting resonators.
Both fields have made remarkable progress and demonstrated a diversity of effects. Given that the different regimes of the same phenomena can be investigated with two different set-ups makes comparing results and sharing insight particularly valuable. The EU-funded project CCQED (Circuit and cavity quantum electrodynamics) brought together premier scientists in both fields from academia and industry. They joined forces and inducted 14 PhD and postdoctoral students into their network to investigate QED.
In addition to training at host institutions, the network provided schools, workshops and meetings to broaden perspectives and share insights on experimental methods and theoretical descriptions. Fellows also organised two meetings titled Young European Scientists in the project's last two years. A remarkable aspect for researchers' career advancement was access to the wide technological diversity of experiments. These included cold atom manipulation, ion trapping, cryogenics, superconductivity, vacuum physics, clean room, laser technology, hardware and software development, and electronics.
Another notable scientific outcome was the possibility of building hybrid systems by coupling superconducting transmission lines to real atoms. Combining these systems could be useful for future quantum computation architectures. The fast superconducting circuits could be the key element for quantum information processing, whereas the real atoms can serve as the basic storing unit.
Researchers produced over 90 publications published in peer-reviewed journals. Some of the most significant results were coherent manipulation of Rydberg atoms on a superconducting atom chip and coherent coupling to a transmission line; squeezed light from a single atom; and all-optical switching from ion crystals. Other results include simultaneous readout of multiple artificial atoms in circuit QED; quantum-state tomography and reconstruction of quantum microwave fields generated by superconducting circuits; and entanglement detection and squeezing of propagating microwaves.
By forming a new generation of young researchers both in academia and industry, CCQED has fuelled the growth of quantum technology in Europe.

The Initial Training Network Circuit and Cavity Quantum Electro-Dynamics (CCQED) has bridged two research areas in physics which both investigate the strong coupling between light and matter at its most fundamental level of elementary quanta. In this regime one or a few atoms strongly interact with a single mode of the electromagnetic field stored in a resonator containing only a small number of photons. This research area, named cavity quantum electrodynamics, has been at first investigated with real atoms coupled to microwave or optical photons. In recent years it was then demonstrated that the very same physics can be studied in a solid-state architecture named circuit quantum electrodynamics, where now artificial atoms made of Josephson junctions are coupled to on-chip superconducting resonators. Both fields made spectacular progress in the past years, with a remarkable diversity of demonstrated physical effects. This was recently acknowledged by awarding the Nobel Prize in Physics to Serge Haroche (jointly with David J. Wineland), whose group was part of the CCQED network. While circuit and cavity quantum electrodynamics share the same concepts, they explore different regimes with fundamentally different techniques. This complementarity has implied a strong motivation to bring together the solid-state circuit and the atomic physics cavity groups in Europe to form a unified scientific community. Hybrid systems, combining the advantages of a real single atom with a superconducting transmission line cavity, are a prime example of how this union might spark new scientific directions.

Network-wide meetings and conferences have strengthened existing links between CCQED partners and intensified the exchange between academia and the private sector. Results and benefits of network-induced collaborations that go beyond the initially planned joint projects, were shared, pursued and diffused within Europe by opening network events to external researchers and by publications in high-impact scientific journals. As an Initial Training Network, CCQED provided education and training with 12 Early Stage Researcher (ESR) positions in a 3-year PhD-programme and with 2 Experienced Researcher (ER) positions in postdoctoral studies during 2 years. Each of the fellows was enrolled in a challenging research project at his host institution, that either dealt with state engineering of two or more particles, with engineering of photonic states or with new technological tools. The high quality of these projects is documented by the scientific and technological outcome achieved during the four-year duration of CCQED with more than 90 peer-reviewed publications. Highlights include, observation of sub- and super-radiance in cavity and circuit QED systems, generation and tomography of Dicke-states composed of multiple superconducting qubits, coherent manipulation of Rydberg atoms on a superconducting atom chip and coherent coupling of Rydberg atoms to a transmission line, development of the open-source C++QED simulation tool to model dynamics of open quantum systems, study of self-ordering of a few emitters as well as quantum dynamics of a Bose-Einstein condensate in a cavity, cavity electromagnetically induced transparency (cEIT) and all-optical switching using an ion crystal, observation of squeezed light from a single atom excited by two photons, cEIT and antiresonance phase shift in a single-atom-cavity system, design and fabrication of a two-mode cavity for studying quantum Zeno dynamics in circuit QED, simultaneous and efficient readout of multiple artificial atoms in circuit QED, quantum state tomography and full state reconstruction of quantum microwave fields generated by superconducting circuits, detection of entanglement and observation of squeezing of propagating microwaves, a frequency comb laser with sub-Hertz linewidth, and a study of current limits of real-time simulation for quantum optics systems.

Apart from the host-based training associated with working on these cutting edge topics, network-wide training through schools, workshops and meetings broadened the fellows’ perspective. They also provided the possibility of fruitful exchange of ideas between the two communities, and the acquisition of expertise in both, experimental methods and theoretical concepts. A key aspect for the career advancement of the fellows was their access to a wide technological diversity of experimental methods: manipulation of cold atoms, ion trapping, cryogenics, superconductivity, ultra-high vacuum techniques, working in clean room facilities, laser technology, hardware and software development, and designing customized electronics. The schools organized by the consortium in the second year of the project introduced the fellows to the basic principles of cavity and circuit QED, as well as to real-time experimental control and high-performance computing. Hands-on lectures given by industry partners and supplementary skills training complemented the scientific lectures. Fellows also had the chance to shape two meetings - called Young European Scientists (YES) Meetings - in the third and fourth year of the project after their own needs. They decided on the scientific programme, the supplementary training and took care of the entire organization and implementation by themselves.

Highlight events of CCQED were the two conferences in the third and fourth year of the project. At the Conference on Resonator QED in 2013 more than 140 scientists came together, among them world-leading experts in circuit and cavity QED as well as closely related fields. CCQED partners and guests discussed the conceptual similarities of circuit and cavity QED as well as the specific advantages of each system. All fellows had the opportunity to present their project to the scientific community in either a talk or a poster. Due to the huge impact of the conference, a follow-up event is planned for 2015. The second conference was the CCQED Final Meeting, at which the scientific and training outcomes of the project were summarized and reviewed by the consortium. Each fellow reported on his research project and training activities. A few invited speakers complemented the scientific programme and took part in the discussions about the results of CCQED.

While all conferences, meetings and schools were open to researchers from outside the network (more than 170 in total), a Fellows’ Day before or after network events was held exclusively for CCQED fellows. The purpose of this event was to strengthen the interaction and communication between the fellows, provide them with additional supplementary skills training and gives them the possibility to discuss and plan upcoming network events.

Four key aspects illustrate the potential societal and economic impact of CCQED and how it has strengthens both, the European economy and the European research community:

First, excellent training of young physicists provided through the network makes them highly skilled future employees of European companies, universities and research institutes, from which both, the European public and the private sectors will strongly benefit. A fellowship at CCQED is not limited to fundamental research but is extended through the strong engagement of industry partners also to applied science. Furthermore, CCQED is strongly committed to provide its fellows and young scientists from outside the network with supplementary skills such as project management, presentation and software training which are crucial for their future careers in industry or academia.

Second, the involvement of private companies directly links fundamental research and applied science and leads to a permanent exchange of ideas and knowledge between those two sectors. As an example, CCQED’s industry-academia collaborations support the further development of the frequency comb, resulting in a marketable product with the potential of broad impact. Through the dialogue with researchers, companies become aware of the scientists’ particular needs, and scientists profit from customized products.

Third, CCQED is convinced that the fascination and impact of its research as well as the general benefits stemming from European research funding shall be visible to society. For this reason CCQED organized a public evening lecture on cavity QED, and a panel discussion in which the relation of innovation and basic research was discussed with representatives from research, industry and the EU-Commission. Moreover, CCQED followed the invitation of the BMBF (German Federal Ministry of Education and Research) to present itself at the national launch event of Horizon 2020, the new European Framework Programme for Research and Innovation. CCQED is also presented in the BMBF official Horizon 2020 brochure.

Finally, through various network events organized throughout the entire project period, CCQED created social and professional bonds between people from different European countries and thereby helped to strengthen scientific collaboration and the exchange of knowledge in Europe.

To summarize, CCQED pioneered powerful new approaches for the study of quantum coherence and, by forming a new generation of young researchers both in academia and industry, it has fuelled the growth of quantum technology in Europe.